You might try these sites.

http://www.moreawesomethanyou.com/smf//index.php

http://www.variousimmers.net/

deene

If you really want to do this (O.o), you could doctor it with the
Insiminator. Break family ties for those two sims, have them woohoo,
and then reapply the relationship. You'd need the family tree, which is
chair->spawn->more (and maybe 'more' again, I can't remember).